Subfoveal choroidal blood flow in response to light-dark exposure.
PURPOSE: To document the response of subfoveal choroidal blood flow (ChBF) in the human eye induced by light and dark exposures and provide some insight into the mechanism underlying this response. METHODS: In a group of 12 volunteers (age, 25-60 years), ChBF was measured with a confocal laser Doppler flowmeter. Wavelength of the probing laser beam was 785 nm (90 microW at the cornea). ChBF was recorded in room light, in darkness, in room light after dark adaptation, and during strong green light exposure after exposure to room light. After dark adaptation of both eyes, ChBF was also measured in one eye while only the fellow eye was exposed to strong visible light. RESULTS: Although ChBF was stable during room light condition, it decreased significantly by 15% (P < 0.01) during dark adaptation. After 6 minutes of room light following 20 minutes of darkness, ChBF was back to baseline. Strong, diffuse, green light exposure over a field of 40 degrees, as well as the probing laser beam, had no detectable effect on ChBF. No change in ChBF was detected when the fellow eye was illuminated after both eyes had been dark adapted. CONCLUSIONS: The findings did not confirm the presence of an active process of ChBF regulation in response to light exposure in humans. They demonstrate, however, a reversible decrease in ChBF that occurs after a transition from room light to darkness, which could involve a neural mechanism.